Hi,
Although Forest objects can be nested, instancing is applied only at the "top level". Any "inner objects" are converted to meshes. In the case of having an "inner object" with many polygons, the Display > Max faces limit (5 million faces by default) might be reached. Raising such a limit should fix it. Let me mention that depending on the render engine (and its release), the Render Mode for the used "nested object(s)" might have to be changed from Automatic to Meshes under the Display roll-out.
I hope that helps.
Best regards,